Me and some friends were building redstone circuits and powered rails and suddenly the server crashed saying

[SEVERE] Encountered an unexpected exception t
t: Ticking entity
at net.minecraft.server.MinecraftServer.r(SourceFile:464)
at hq.r(SourceFile:248)
at net.minecraft.server.MinecraftServer.q(SourceFile:395)
at net.minecraft.server.MinecraftServer.run(SourceFile:329)
at gi.run(SourceFile:565)
Caused by: java.lang.OutOfMemoryError: Java heap space
at dt.<init>(SourceFile:62)
at it.l_(SourceFile:151)
at yv.a(SourceFile:1312)
at iq.a(SourceFile:480)
at yv.g(SourceFile:1290)
at yv.h(SourceFile:1197)
at iq.h(SourceFile:374)
at net.minecraft.server.MinecraftServer.r(SourceFile:460)
... 4 more
[SEVERE] This crash report has been saved to: /Users/Roberto/Documents/Minecraft/SERVERS/MontanhaRussa/./crash-reports/crash-2013-01-18_18.44.34-server.txt