Lightning may be right. But, before we try that, let's check a few things. I have Access 2002 (XP) but 2000 is probably similar. Do this:
First, make sure you have a copy of your new Workgroup file somewhere.
Run Access.
Click on the tools menu and then select Security..., Workgroup Adminstrator...
What workgroup is shown?
If the wrong Workgroup file is shown. Click on the Join... button and join the correct workgroup file.
Click OK to close the Workgroup Administrator window.
Click on the tools menu and then select Security..., User and Group Accounts...
Do all your user and group names show up here?
If you already had the correct workgroup file selected and all the user and group names are OK. Then we can try plan B, otherwise try opening you database again.
PLAN B
If you got an error in the above process or the above did not correct the problem, then try this:
Make sure you have a copy of your workgroup file!
See if you can compact and repair the workgroup file (MDW) file. (The workgroup file is just another database file).
Open Access
Select tools, Compact and Repair Database....
Set Files of type to 'mdw'. Compact your existing mdw file into a new 'mdw' file.
Any Errors?
Then rename original mdw file, and give the newly created mdw file the original workgroup file name.
Try opening your database again.
If this doesn't work and you can't re-create your workgroup file from scratch, as suggested by Lightning, then maybe we can come up with a plan C...