Ps3 wont play blu-ray but will play dvd

Maybe your PS3 Blu-ray playback partition has been damaged. But you can convert the Blu-ray to regular video and transfer to PS3 for playback.
Leawo Blu-ray Ripper is an easy-to-use Blu-ray to PS3 converting tool to help you rip and convert Blu-ray file to PS3 videos in only a few simple clicks. The below step-by-step guide would tell you the details on how to rip and convert Blu-ray to PS3 videos with Leawo Blu-ray to PS3 Converter.
Guide on how to convert Blu-ray to PS3 videos:
Step 1: Download and install Leawo Blu-ray to PS3 Converter, and then launch it.

Step 2: Get source Blu-ray movies ready. Insert Blu-ray disc to the Blu-ray drive, or get source Blu-ray folders ready for loading and converting.
Step 3: Load Blu-ray sources
Click "Add Blu-ray/DVD" button on the program interface. This Blu-ray to PS3 converter would then guide you to browse source Blu-ray movies for loading.
Note: After loading source Blu-ray movies, you could select subtitles and audio tracks, play source Blu-ray movies, edit Blu-ray movies, snap screenshots, etc.
Step 4: Do output settings

Next to the "Add Blu-ray/DVD" button, click the dropdown box and select "Change". You will then arrive at the Profile setting panel to select output format from "Format" group or "Device" group. Select "MP4? from "Format" or directly choose "PS3? from "Device > Sony".

Optional settings: After you've set the output format, click the same dorp-down box and click "Edit" option and then you could adjust video and audio parameters of selected profile, including: video codec, audio codec, bit rate, aspect ratio, frame rate, channel, etc.
Step 5: Start converting Blu-ray to PS3

Back to the main program interface, click the green "Convert" button. You could then set an output directory on the right popup sidebar. After setting output dirctory, click the below convert button to let this Blu-ray to PS3 Ripper instantly start converting Blu-ray to PS3 videos.
Note: Internet connection is required during the whole conversion.
After you convert Blu-ray to PS3 friendlier videos, you can transfer the videos to PS3 and then start to play Blu-ray on PS3 in a indirect way. For details, you can read the extension part.
After the conversion is completed, now we should transer Blu-ray movie now in PS3 fridlier video format to PS3 for playing Blu-ray on PS3. Follow the steps below to do so.
Here is a simple step-by-step guide for transferring to or watching video files on your PS3 using a USB flash drive or an external hard drive.
Step 1: Plug a USB flash drive or an external hard drive into your computer and copy over the converted video files that you want to watch on your PS3.
Step 2: After the copying process is done, pull out the USB flash drive or the external hard drive from your computer and then plug it into one of the USB ports on the PS3.
Step 3: Navigate to "Video" on the PS3 XMB menu and find your device, hit triangle to view all contents and select the file you want and either copy it to the PS3 hard drive or just play it directly from the connected device.
By following the guidance above, you can achieve your goal of more safely and conveniently playing Blu-ray on PS3 or watching Blu-ray on PS3 for protecting either PS3 Blu-ray drive or your precious Blu-ray discs from being scratched.

It's probably because the blue laser is wearing out, you might need to replace it to get your PS3 working again. Here's one way to play Blu-ray movies on PS3 without needing the blue laser. Direct and Indirect Ways to Play Blu Ray on PS3 Respectively

Ps3 drive and blu ray drive inside of your ps3 went out. try cleaning them and if that doesn't work replace them or send it to sony.