There might be a lot of causes why you are not able to access your VPS hosting server. Common mistake is that customers think that VPS server is down when they cannot access their websites anymore. This may happen just cause Apache web server is being down and a simple Apache restart will fix the problem.
Other reason why a VPS hosting server appear offline is because allowed hardware resources are being maxed out temporarily or continuously. Due to VPS hosting memory allocation limits most of the time. In such a case this may mean it is necessary for you to upgrade you VPS hosting server to the next VPS hosting plan or dedicated server or just add additional guaranteed memory (RAM).
If you still have doubts why your VPS server is crashing you can just submit a support ticket and request analysis of your VPS hosting server. We will see what is the issue with your VPS and get back to you with the best solution possible.