According to my calculations based on the Gdax exchange, BTC dropped just over 70% from the high last December. The good news is that yesterday may have been the low for 2018 and it should rebound fairly quickly IMO.
I haven't read the comments above, but just hodL and you should be OK. Look at the longer term, like a few years from now. BTC has had some massive selloffs over the years similar to the one we just experienced. The crypto market is one of the most volatile markets around.
You may want to look into and learn basic technical analysis before investing again.