Unable to fork: Cannot allocate memory vzctl enter 1212

We get this error while trying to enter VPS from hardware node when VPS gets hang or it is running our of memory space.

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[root@server ~]# vzctl enter 1212
Unable to fork: Cannot allocate memory
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run the command to check the disk space in the VPS.

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vzctl exec 1212 df -Th
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If disk space is full increase the disk space of VPS and it will start to work and If disk space is not full just reboot the server it will start to work also if while running above command if you get the same error reboot the server and then give it a try.