This could be so many different things. First, try running down my Troubleshooter. If that doesn't fix the problem, come back to me and we'll try some more stuff.

Also you should try to get your users to work with the tables through a form and not have them working inside of tables.

In addition to what Richard said he has a great video on the second part of your question.

Record Locks
AND
In my opinion, even better is the
For me this template was worth it's weight in gold.

Thank you for your response, and yes I am getting the error within the form part, not my table.

Since you are sharing your db - Make sure each person is using their own copy of the front end.  See Split Your Database if you don't know what I'm talking about.  

1 Close down Access and make sure everyone else is out as well.
2 Reboot your computer and theirs.
3 Go into the folder where you have your db and look for the .laccdb file.
    (If you don't have file extensions on do a search on how to turn them on.)
4 Delete the .laccdb file:   [YourDatbaseName].laccdb

That will get you back running but my concern is why you got stuck in that state.  Could be many reasons.
Keep an eye on your db and see if it happens again.  If it does try and find out EXACTLY what the chain of events were to get  you there.