The answer could be more specific, if you let us know, whether it is playing other videos from other sites.
If yes, then it has to be the issue of video format support. If no, then there can be few more reasons like:
1) Memory issue, you need to free some memory.
2) Check your internet pack, if it has exceeded the data
3) Might be some virus catching up your device
So, to ensure all the above,
a) check your phone's memory status
b) check your internet data pack
c) you need to load and run a good antivirus.
Hope that works.