This is not a big problem and server related error and have personally encountered it few days back while opening the forum and as you have been able to load it after few hours mine was fast comparitively but this is not a big problem.
There are many reasons for this like @hugeblack have already told you 5xx is the general code for these sort of errors and to add i would say that 200 is the code for that each action was successful but as everything is loaded we can't see this message displayed over our screen.But 400 and 500 are the codes for server related errors
But if you are interested to find out specific errors with the help of decimal point then here is the list for you to have a look at :
However, if the server has installed Microsoft Internet Information Services (Microsoft IIS), the error code will be specified. Decimal places indicate the cause of the error in more detail:
500.0: Module or ISAPI error occurred.
500.11: Application is shutting down on the web server.
500.12: Application is busy restarting on the web server.
500.13: Web server is too busy.
500.15: Direct requests for global.asax are not allowed.
500.19: Configuration data is invalid.
500.21: Module not recognized.
500.22: An ASP.NET httpModules configuration does not apply in Managed Pipeline mode.
500.23: An ASP.NET httpHandlers configuration does not apply in Managed Pipeline mode.
500.24: An ASP.NET impersonation configuration does not apply in Managed Pipeline mode.
500.50: A rewrite error occurred during RQ_BEGIN_REQUEST notification handling. A configuration or inbound rule execution error occurred.
500.51: A rewrite error occurred during GL_PRE_BEGIN_REQUEST notification handling. A global configuration or global rule execution error occurred.
500.52: A rewrite error occurred during RQ_SEND_RESPONSE notification handling. An outbound rule execution occurred.
500.53: A rewrite error occurred during RQ_RELEASE_REQUEST_STATE notification handling. An outbound rule execution error occurred. The rule is configured to be executed before the output user cache gets updated.
500.100: Internal ASP error
But as user side you can simply come back after some time and the site will be displayed as most probably the reasons are huge traffic at the time or memory overusage but this can be sorted after some time.So don't need to worry about them as they are not major issues and automatically resolved.
But still from your side try clearing cache is the best you can do at the moment you face this error and reload the site then again and it might load the site then.But if you want some more information check this :
HTTP 500 server error guide 
