Partition failed but is still taking up space

I have programs on my MB that won't work on Lion so (since I had already installed Lion) I decided to partition my 500 GB internal hd and load Snow Leopard onto the partition. When I tried to create the 249.62 GB partition the process was interrupted. Now, although it is showing that there are no partitions, there is 249.62 GB missing. In the disc utility it shows:
Capacity- 499.25 GB
Available- 101.13 GB
Used- 148.49 GB

Is there any way to get back the 249.62 GB?

Also, whenever I try to create another partition it now says "Couldn't modify partition map because file system verification failed."

Any help would be appreciated.

You can try this:

First, reset the PRAM.

Next, boot the machine with your install DVD or Lion DVD (if you made one) and go into Disk Utility. You're probably going to see two partitions - Lion and its recovery partition. Since I'm not sure how you attempted to make the additional partition for SL, it may be after the recovery partition. If it shows up like that or the unallocated space does... there's not much you can do but start over. The reason is you can no longer merge the partitions because of the recovery partition.

Let us know and we can go from there. But I suspect you're going to need to erase the drive and start over. Make sure you have backups.