Its not just ComStar as the big bank- you know there was probably a Bank of the Commonwealth, TharkadBank, Marsden Bank, etc but its ComStar's quick interstellar communications that allowed human-space wide transactions to quickly occur. Its more like the spine of the transaction network collapsed- instead of going into the store, presenting your credit card, running it through a machine, getting notification of payment & recording a signature . . .
Think back to when the cashier pulled out the little sliding copy device. The cashier would put your card on the proper space, fill out the triplicate form for the purchase, tax and total, put the triplicate form on top of your card & the device, swiped the imprinting bar so the triplicate has the card information, and check the signature against the back. IF its a big enough purchase, by your store's policy, you call the CC issuing company's number for a proper authorization and also copy that number onto the triplicate form. Then you give them the bottom . . . yellow? form, and put the top (white) and middle (pink or yellow) forms in your drawer. End of the day, you keep one copy (white) to turn in with the records and the other goes into the bank deposit bag where IIRC the bank uses the wire service to transfer the funds into the company account . . . which IIRC originally could take weeks. And could bounce like a check, which is why you got authorization if it was a large enough purchase.
Without ComStar's sphere-wide communication, its not just the prevailing currency that fails its also the monetary transfer system. Unlike before Gray Monday, I cannot be a merc on Podunk that places a order for 5 tons of Thunder LRMs from a factory on Tikonov over 200ly away through the HPG with payment in escrow through a ComStar account. Finding out in a few days to a week that my order is accepted, based on exchange rates & market fluctuations, I got X back from the escrow account, and my order of 5t will arrive in two months taking blablah chain of JS/DS to arrive on Podunk. Not being fast enough, I try to get the shipping company to transfer the to make part of it command circuit-like though I will have to pay express rates it might be able to make the trip in a month and a half. Never mind that most businesses like mercs are going to hold their money in C-bills since it was the more stable currency, more widely accepted and if placed in ComStar bank you had access in a variety of places- lots of international operations would have started a run on the c-bill and ComStar bank when it became apparent it was human-space wide and the HPGs were not coming back. They probably suffered from several runs depending on timing.
Now after Gray Monday . . . if I want to be certain I can get the 5 tons of Thunder LRM, I better send a purchasing agent who will take around 2 months to get to the system. I am not going to have any idea how long its going to take b/c there is no way to circulate JS schedules since nothing is guaranteed until the ship shows up. If I was in the Republic in the early 30s, they should take Stones (or whatever) but if I know the Swordsworn is running the world that might be questionable . . . I might want to send someone with D-bills. So do you send enough Stones to buy what you want along with enough D-bills? How about after the Fortress goes up (and I am outside), I am on a former Republic world, and Aaaron Sandoval has taken Tikonov into the FedSuns? Stones will have become worthless on worlds outside the Fortress the moment Levin makes his announcement (think Weimar Republic levels of inflation). Who/how would I even exchange my Stones for D-bills with on Podunk, since previously it was ComStar that handled currency exchange options- and kept the foreign currency on hand. So . . . currency is out, my purchasing agent boards the dropship with a cache of silver, platinum, and industrial grade diamonds. The purchasing agent arrives three months later on Tikonov during a Liao raid to find out the production from that missile ammo factory I was going to buy from had been nationalized for 'the duration of the emergency' and to top it off, the Davion customs agent seized the agent's cache of silver, platinum and industrial diamonds as 'strategic resources' with a voucher for their value in D-bills from some agency on planet. Any Stones the purchasing agent had are absolutely worthless on Tikonov (he discovers the blackmarket is also accepting L-bills!) and when the voucher (hrmm, let me see that . . . looks like a fake to me) is finally paid by the agency, the valuation is half what it was on Podunk b/c of scarcity/market forces/wartime nationalization.
Interstellar commerce is hosed after Gray Monday, and the merc trade relies on it. When the Fortress announcement went out, all the worlds outside should have gone into absolute chaos with shades of Weimar and Venezuela.