First, check to be sure you're using the right number. Your barcode number is 14 digits long and will start with '28153'.
If you are using the correct number and it still isn't working, you'll need to contact a librarian to have your account re-activated. Accounts are reset every semester, and a number of things, including transferring and late registration, can cause your account to be missed.
Just give us an email at library@ muskingum.edu, call us at x8152, or talk with us via chat and we'll get your account fixed ASAP.