It sounds like you may have inadvertently set calculation to Manual.
If you'll click on the menu Tools Options then choose the Calculation
tab; select the radio button for Automatic.

By the way, you might consider this MID function: it uses a FIND to
locate the first period. This may be useful to you if that period
doesn't always show up in column 5.
=MID(C3,FIND(".",C3,1)+1,5)