I recently purchased an external hard drive for my laptop.  It worked
fine initially, but after I tried to install Ubuntu Linux, I must have
erased some data the dealt with the partitions.  My external harddrive
has the power on, and is connected to my laptop, my computer even
recognizes it by giving me some USB cable message when I plug it into
my computer, but I have no way of getting anything on the hard drive. 
I can't open it on the my computer menu.

Are you still running Linux or are you running Windows?

What leads you to believe you might have wiped the partition table? 
Was the drive plugged in when you installed linux?  Do you think you
may have perhaps put a linux filesystem on it?  If there is a problem
with the partition table, the data on the drive is most likely toast
and will need to be repartitioned and reformated.

If you're running Windows, try right clicking on my computer and hit
manage, and look under the disk administrator.  If the drive is not
partitioned correctly, it may show up there and you can repartition
and reformat it from there.

I am running windows XP but I can run Ubuntu Linux through a CD.

I looked in the management disk administrator thing and saw a 730MB
partition which I deleted, I still can't access the rest of the disk
and I have no way of reformatting it.  It won't let me open it if I
try right clicking on it.

Thank you very much for your help.

You need to partition it through the disk administrator.  Once you
delete all the partitions it should indicate one large block of free
space.  Right click it, partition, and then format it.

thanks again.

Are you telling me that I can't get any of the old data off the external drive?

If the partition table is messed up, it is very unlikely.  

There may be some utilities out there that will attempt to do it, but
I wouldn't hold my breath.

You said you deleted one partition, what does windows say about the
other?  Is it not recognized?  If so, it may be a linux partition. 
Did you store the data on the drive while in Linux or while in
Windows?

If you stored the data on the drive while in Windows, and Windows does
not recognize the partition, the data is most likely gone.  You could
try mounting the drive in Linux, to see if it sees anything.

may also have luck using software like EasyRecovery Pro to get some of
the data off, but it's expensive to buy (& expensive to pay someone
who already has it), so that's probably only worth pursuing if the
data is really worth it?