There are several reasons as to why you may not be able to access your VPS. Some users often mistakenly think the VPS is down when they cannot access their websites. This could be due to apache being down which may need a restart.
More often than not however, a VPS may appear offline because resources are continuously being maxed out, most likely due to memory allocation limits. In this case it may be necessary to upgrade to the next VPS plan or add additional RAM.
You can open a support ticket to request for a analysis of your VPS to see what the issue could be.