Have you tried turning your drone back on? This may fix it. If you do a search here you should also be able to find a post for a utility that allows you to "fix" the open file
Read the manual .... it has a line specifically about this ...
Page 47 in the FAQ ...... for the P3 Standard
Page 51 in the FAQ ..... for the Advanced and Professional
Have SD card in the slot and turn on AC ... wait about 30secs and video should be finalised. Watch the little LED on the gimbal ... when steady - should be ok.