A village post office could not pay pensions or benefits for 11 days over Christmas after its broadband connection was cut off.
Sub-postmistress Wendy Telfer offered to lend her own money to any "really desperate" customers of the branch in Bellingham, Northumberland.
"It's the busiest time. People wanting to send parcels and get their money," she said. "It's really affected us."
BT is investigating what happened and why the reconnection was delayed.
The Post Office has apologised to customers for any inconvenience.
Mrs Telfer said two customers had taken her up on her offer.
The internet connection was lost on 17 December after BT installed broadband in the upstairs flat, Mrs Telfer said.
She said the company assured her the new connection would not affect the two lines in the shop and post office below.
But until Friday, the branch could not process any payments, including council tax and other bills, or any other service which uses the computerised system.
Mitch Reid from BT said: "On the face of it, it looks like the two matters are linked, but I can't confirm that."
He said the company was also trying to find out why when an engineer returned on 18 December to install a new line in the flat, the post office connection was not restored.
Mrs Telfer said she had only been able to sell stamps, writing down the transactions in a ledger.
This has affected her income, she said.
"Whatever goes through the system I get paid on. Put nothing in, I get nothing back.
"In this day and age, you know, it's absolutely ridiculous."
There are post offices in nearby villages but Mrs Telfer said many older people relied on the Bellingham shop.
"A lot of old people haven't got bank accounts. They're having to rely on their pension. I did say if you're really desperate, stuck, I'll lend you it until I get going, out of my pocket."