I'm here to share some information so you'd be able to access your QuickBooks account.
QuickBooks Online and QuickBooks Desktop are two different QuickBooks products under two different platforms. You should still be able to access your QuickBooks Desktop company file after the conversion process. Just to clarify, what specific error prompt did you received when trying to open your QuickBooks Desktop company file? This would be very helpful so we can narrow down the root cause of this concern.
In the meantime, here are the possible reason as to you're unable to access your QuickBooks Desktop account:
You may have corruption on your hard drive
Damaged or missing QBWUSER.INI file
Your QuickBooks Desktop installation may be damaged
The company name is too long
To resolve this, we can download and install the QuickBooks Desktop Tool Hub. This is a great tool that can help isolate possible data damage in your company file.
Step 1: Download and install the Tool Hub (Helps fix common errors)