Difficult to say without knowing what you tried that was successful.
In general, for Reference problems what you need to do is open any code
module (or simply the Debug window, using Ctrl-G, provided you haven't
specified "Debug Window on Top" on the Module tab under Tools | Options),
then select Tools | References from the menu bar. Examine all of the
selected references.
If any of the selected references have "MISSING:" in front of them, unselect
them, and back out of the dialog. If you really need the reference(s) you
just unselected (you can tell by doing a Compile All Modules), go back in
and reselect them.
If none have "MISSING:", select an additional reference at random, back out
of the dialog, then go back in and unselect the reference you just added. If
that doesn't solve the problem, try to unselect as many of the selected
references as you can (Access may not let you unselect them all), back out
of the dialog, then go back in and reselect the references you just
unselected. (NOTE: write down what the references are before you delete
them, because they'll be in a different order when you go back in)
When you say that "after a few hours the problem occured again", what else
happened during those few hours? Did you install any other software? If the
database is on a server, did you close it, someone else used it and then you
went back into it during that time?