Some (most?) M$ software is a bit hostile to allowing access to data files while it is being worked on.
Since you rebooted while the program had a file open it was probably stilled locked.
Not sure how Powerpoint locks its files but you could try checking the 'read only' flag.
Right click on the file & select 'Properties'. There will be set of tick boxes that allow you to change the read only, archive, hidden & system flags. Make sure that the hidden, system & read only options aren't selected.
Alternatively, if Powerpoint is like Word or Access, it creates a temporary file for each document opened. Perhpas you could try deleting or changing the read only flags on this file?