Open Excel and go to Preferences, then click the Edit tab, and uncheck the box "Fixed decimal"
That should do it
Please feel free to reply if you need further clarification or click Accept if this answers your question fully.
Someone must have checked that box (or it may have been checked by a macro that you may have run recently)
As for the reason, some users prefer this option for faster data entry. For example if you use Excel to enter a long list of dollar amounts frequently, instead of typing a decimal in each entry you can set the option on 2 decimals and it will automatically enter the decimal for you (e.g. instead of typing 22.95, you just type 2295)
Hope this helps