Hello Seamii and welcome to Seven Forums. Couple of things you can try.
If this just started you could try rolling back your computer to an earlier date/time using system restore. Since sticky notes are a part of the Windows 7 system files they might have gotten damaged or corrupted in some way. System Restore
If this doesn't work (or you don't have any restore points) you could manually check for damaged or corrupt system files by running a system file checker scan from an elevated command prompt (option two.) If problems are found, run the scan 3 times and reboot the computer after each scan. SFC /SCANNOW Command - System File Checker
You could also see if there are any application conflicts that might be interfering with sticky notes. Troubleshoot Application Conflicts by Performing a Clean Startup