Its a real vague report.
It seems you are playing your mugen with tons of characters OR some characters but really heavy in size and those characters are directly being loaded from your RAM (its the bad part of mugen) and that explains why its delaing to load.
For your sudden crash error during character selection it may be related to the character itself due:
1- bad logic coding
2- debug overflow due coding error or lack of proper logic checks that slighty helps in ram consuming (people dont believe that but debug errors slighty contribute in lagging mugen cause mugen is being overloaded with working due in how the files are being read in less than a millissecond every time so they leave debug errors active cause the "char is still playable" or even doesnt know how to fix their own errors). Now... if your mugen roster is composed with tons of characters that have debug errors then.. ouch. You either ask the author to fix them or you fix yourself.
3- The character is intended to crash mugen without any sign of errors if you change the authorname or some code within it as its possible to confuse mugen in avoiding telling you where the error is located and it will crash without saying one.
4- Direct X related, but i do think if this was ever the case your 1.0 wont load at all.
5- Your RAM is being consumed too much in a point your own PC is saving resources and neglecting mugen to execute stuffs, which will cause crashes.
Theres also a lot of possible errors that i havent included so unless you are specific then nothing can be done.