nc50lc is probably right, an other option might be the conversion rate.

If you bought the BTC at a time when the conversion rate to USD or EUR was higher than it is right now, it might look like 20% of your funds are "gone" when you look at the FIAT equivalent.

1 BTC today is the equivalent of ~$9100 at this point in time. It's possible that in a couple weeks, your 1 BTC is worth only ~7300, so it looks like 20% of your funds "dissapeared". But in reality you still own 1 BTC, it's value just fluctated.