How to fix cannot contact the license server in Citrix XenApp

Hello people!
Being an inexperienced chap in working with Citrix, I need the help of experienced people out there. Every time after logging in to Citrix Server, a message pops up “Citrix XenApp cannot contact the license server”
The services become inaccessible after some time.
Please help in fixing cannot contact the license server in Citrix XenApp error.
