Without having so much as an indication what version of Office you're using
it's impossible to suggest anything other than the fundamentals;
1- Make sure your version of Office is fully updated,
2- Make sure OS X is fully updated,
3- Run Disk Utility - Repair Disk Permissions,
4- Restart your Mac.
If you continue to have a problem reply here with version specific
information plus any additional details you can supply. For example: Does it
happen with all documents or just some? Has anything els changed recently on
your Mac? Is this a new installation or one that has been running OK
previiously?
Every situation is a bit different so without adequate information there
isn't much anyone can do to help you. If there were a univeral "Fix This"
button you would have found it by now