Quickbooks won't let me login to one of my companies

To help us narrow this down, could you share the specific error message, whether you are in single-user or multi-user mode, and if it affects only one user?

In the meantime, you can troubleshoot this by right-clicking the QuickBooks Desktop icon and selecting Run as Administrator. Next, run the Quick Fix my Program tool from the QuickBooks Tool Hub, and make sure that your company file (.QBW) hasn't been moved or renamed from its original location.

Keep me posted on how it goes or if you have any other questions at your earliest convenience.

​@Cindy Yackobovitz What error message?

What error message?

What error message?