Something may be preventing PostgreSQL from starting when your computer does so if you're using any security software (such as antivirus, firewall), please make sure to follow the instructions given in the
Security Software Configuration Guide.
Next locate and delete the postmaster.pid file if it is present (not "postmaster.opts" which may be there too). You can find that file in C:\Program Files\PostgreSQL\9.3\data\ although you may have a different PostgreSQL version (e.g. 8.3, 8.4) or have it installed in C:\Program Files (x86). Once it's deleted, reboot and see if that resolves the trouble.
If that does not help then click your Windows Start button then type the following command:
services.msc
and click on it when it appears. Do you see the PostgreSQL service in the list? Is it started, or can you start it?
Finally go to "Control Panel -> Administrative Tools -> Event Viewer -> Windows Logs -> Application" and in the middle open any recent PostgreSQL items with errors and let us know what they say.