The file association has gone missing I reckon.
Shift + right click on an mpeg and select Open With from the pop-up menu. Select your preferred player from the list presented and make sure the box "Always use this program..." is ticked.
Surface Pro 5 News - release date, UK price, features, specs
Microsoft Surface Studio hands-on review: the iMac killer is here
Best Mac antivirus 2017
Synology invites you to visit them at Insomnia on 26th – 29th August. Book your tickets now and visit Synology.