Lost the ability to add anything to my calendar on Apple 4S

[sweats24]

I have had this phone for over a year and keep many things in  my Calendar app that came with my Apple 4S phone.  Just recently in the past 7-10 days, the bolded "+" sign that allows me to add something is not bolded any longer and I cannot get anything into my calendar.  Have I touched a setting that I shouldn't have, added a conflicting app, I just cannot seem to get it back.  I have gone through everything in Settings but nothing seemed applicable for that issue.  I would certainly welcome anyone's solution.  Thank you.  Tom.

[ttipgem · Accepted solution]

Have you tried a hard reset on the phone?