This is because of insufficient memory.
If in the process of drawing, has not been closed, the cache of the previously opened picture will exist, it will cause the PS cache to run out.
You can save the file and restart PS; You can also open [Edit] - [Preferences] - [Performance] to set the scratch drive to another drive letter; You can also clear PS's cache space.
Here are the details:
1, you can turn off PS, restart it, the cache will be released;
2, the system default PS temporary disk is set in C disk,C disk space is not enough to open [edit] - [Preferences] - [performance], the temporary disk to other free space on the drive letter, you can also select multiple drive letters. Then change [Memory usage] to 100%, and then restart PS to take effect.
3, PS in the case of use can also clear the cache data. Click: [Edit] - [Clean] - [all] to clear the PS cache;
4, you can also adjust the virtual memory: right click [My computer\/This computer], select [Properties], in the [Advanced] option page, click [Settings] in the performance, select the [Advanced] TAB again, click [Change] in the virtual memory, after the setting is completed, restart the computer to take effect.