There are 2 possible causes:
1. The DVD's are degraded due to age and/or bad way of storage. Maybe you made a backup on a more permanent device (like the cloud or two different external hard disks)?
2. The DVD-player doesn't have the right output for the new TV. Do other DVD's play to the the TV?

- Do they still play on a PC or laptop with (a) a DVD-reader and (b) a program like VLC-player that can play DVD's?
- What happens if you look at the contents of those DVD's in Windows Explorer in your PC or laptop? And if you copy the contents to your hard disk? And if you make an .iso-file with IMGBURN (free download)?