Dear Madam/sir,

I would like someone help me. I have one problem with my External HHD. It doesn't show drive letter in the my computer after I convert it to New file system. Then it start to error. Please help me how to fix it. I worry too much about my data. I don't want to loose any. Thanks in advance.

Leave the external hard drive plunged in and reboot the PC if it doesn't recognise the hard drive the go to the website for the maker and download the software they recommend.
If you ask a follow up give manufacturers name and model.

Download and burn Gparted (free) on a cd, and boot on it.
http://www.wikihow.com/Use-Gparted

Or, on windows 7 (might not work), start -> right click on Computer -> manage -> Disk management -> right click on your HDD, add letter...

What file system did you change it too as that may well be the root of your problems ?
Also why did you do this?