cr2504life - if you were flying down a road and backed off the throttle w/out pulling the clutch in, the hot motor is still turnign high rpm, but has no lube or cooling fuel. The big end bearing probably went out, and with all the extra play in the rod wore a hole in the crankcase. That's what happened to my '99 engine.
For mine, a crankcase repair was possible but may have cost $100+. For Honda CR250s, you can buy the crank case halfs individually (around $160 I think from Service Honda) and the new crankcase half will have a crankshaft bearing installed in it (approx $30 part); so it might be cheaper for you to buy a new case. BTW, for the CR250 you can also get a new crank assembly (crank/rod/big end bearing) for about $160 and a new piston & top end kit will be about $120. Add in a gasket set $70+, a second crank bearing ($30) and a few odds and ends ($50) and you are looknig at about $600 for parts, plus tax and shipping; more if you need both crank halfs. If you are going to do it yoruself, you will need a shop manual and special tools, as well.
The prices I mentioned are US$, and not full retail like you might have to pay at your local dealer.