I was driving back to work from lunch, was going 60 then bumped it up 70 and shifted into 5th then a few seconds later hit the clutch again just before I was going to brake to pull up the drive at work and heard a sound like something fell off my car then my clutch wouldn't decompress so I shut it off then pulled the clutch up by hand which wasn't difficult, and tried to start it again and it made a horrible noise so I shut it off

Can anyone tell me what might be wrong with it and what could cause it?

 

Get down by the pedal (easiest if you sit or kneel on the ground next to the driver's seat with the door open) and check to see if the clutch pedal is broken. It should only have smooth motion, zero flexing from side to side, it's motion needs to be completely linear.

If the clutch pedal assembly is broken, this is a known problem, there is an 8yr 100k warranty on it. Otherwise you can get a welded one from BHR for about the cost of a new pedal from the dealership that can fail again. http://black-halo-racing.myshopify.c...d-clutch-pedal
Or you can get a new pedal from Mazmart for far cheaper than the dealer. http://www.mazparts.com/en/rx8-clutch-pedal

If the pedal is fine, then you either had a clutch hydraulics failure or the clutch disc burst.

 

I checked and the pedal is linear how much would either of the other possibilities cost roughly and is there a way to double check them?

 

Basically there are several ways you can verify, and the costs will vary depending on what you can do yourself.

To check for a burst clutch, pull the starter and look into the bellhousing. A burst clutch will leave shreds of clutch material all over the inside, probably even stuffed into the starter nose and probably have some falling out of the hole. At the most, you will have to just look inside.
Repair would mean a new clutch kit, an OEM one is $350 + shipping from Mazmart, $375 shipped from BHR. Labor is free if you do it yourself, or in the $400-$600 range if a shop does it.

To check for hydraulic failure, get under the car and watch the clutch fork on the transmission while someone else moves the clutch pedal. It needs to move in concert with the clutch pedal. If it doesn't stop and start at the same time, moving smoothly through the complete range that the pedal is moving. If it stops when the pedal is still moving, doesn't move at all, etc... then it's a failure somewhere in the clutch hydraulics.
Repair will be dependent on what part of the hydraulics failed, though typically the $200 to replace the line, clutch master, and clutch slave is well worth it. That is parts, labor is free if you do it yourself, somewhere more than that for a shop to do it.