First of all make sure you are not confusing Physical Hard Disk Drive with Partitions.
Assuming that you have 2 or more HDD's and if your hard disk drives seems not to be shown on your file browser, maybe they are unmounted or disconnected.
But having in consideration that all the people is assuming that you erased your partitions when installing, then: yes... you are in the need to start data recovery and probably forensic procedures to do it.
I suggest you to run a search on this site with the words: "data recovery" (without quotation marks) and take a look at the previous experiences by other users.
I also advice you to see this questions in detail, which seems to be working for some users: How to recover Deleted Files and Folders? and How can I recover data from a 2TB hard drive?.
You may also wish to take a look at: https://help.ubuntu.com/community/DataRecovery which contains detailed procedures for recovering lost data.
You need to stay calm and stop working on your computer. The more you do with the computer is the more risk you expose your data. So please use a different computer to find information and if possible, use a live session for the recovery.
I am following your question closely in order to help you. Please let us know how are you managing this issue.