I had the same problem for a long time a while back and went through a lot of diagnostic tests,issues and tools to solve it,like DNS problems,clearing caches,even had my Movistar telecom provider try and figure it out etc. but no luck.I finally solved it with an IP unbloquer tool.The one I used is this one in Spanish
https://hostrentable.ar/desbloquear-ip.php but there are a lot of other free ones on the web.Also I first tried Anonymouse as an external server tool to verify if the problem was everyone or just my IP.Here is that link
http://anonymouse.org/anonwww.html .Just put the photoshopcontest link where it says enter website address and you should get access if the problem is just your IP . Then use an IP unbloquer on your personal IP and that should solve your problem.Even though I have solved the problem I have never found out by whom,what or why my IP was bloqued from PSC.Hope this helps. 🙂