If you just got this from Xmind :
'java.lang.OutOfMemoryError': Java heap space
Stay calm : your file is not dead
It’s a common bug of Xmind 2013 occurring if your .xmind file is too big. The easiest way to save your file is to delete some revisions which are made automatically, and cause the inflating weight of your files.
Step 1 : Back up your file
Step 2 : Open it with an archive software (like winrar), right-click>open with >Choose program… In windows.
Why ? Because an xmind file is in fact a rar archive file containing stull like xml files, images, attachments etc…
Step 3 : Open Revisions folder
Step 4 : Delete all subfolders but the first one (which should be the last revision)
Step 5 : close, and try to open your file – It should work.
I hope it helps.
To avoid further occurring of this problem, delete revisions in a regular fashion from time to time in file>revisions in Xmind.
Note : I love Xmind, but I have to admit that it’s not the most stable of mindmapping software. Obviously, I’ll update this post as soon as Xmind will get a reliable crash recovery. Even if the problem described here is not a big one, some users frequently reports that Xmind crashes while saving process and create a 0 bytes file. Back up your Xmind files regularly.