Are you shown an error message? If so what is it?
What OS are you using?
what versions of Safari, Chrome, Firefox are you using?
are you using any 3rd party utility (clean my mac, mac keeper, little snitch, wire shark, etc.)? If so remove it as per the developers directions on their website.
are you using a mac anti virus solution? If so remove it, see above.
are you using any plug-in that could prevent videos from playing?
boot into safe mode
Try safe mode if your Mac doesn't finish starting up - Apple Support
for Safari and Firefox download flash from this site and only this site.
https://get.adobe.com/flashplayer/
any other site that directs you to a download of flash should be always be avoided.
Chrome has flash built in and you should update Chrome itself.
If the problem is still there post an etrecheck report for further analysis
www.etrecheck.com