Should I stop contributing to 401k until July and August with coronavirus?

No. Keep contributing so that you get the match, but if you don't want a big risk of losing, then put it into a stable value fund or some other low-risk option that your 401K plan offers.

Buy low, sell high.

You asked if you should NOT buy low.

Never heard of buy low, sell high. You might want to do some research about investing.

Right now the markets are down, so the investments you have may be showing losses. This only really matters if you were planning to retire soon. If you have years to work before retirement, the markets will recover and your investments will go back up. 

If you continue to contribute to your 401k right now, your investment should actually go farther in the long run. Consider it this way: you put $20 a week into your 401k, and your employer matches it. This week, your investment account buys 20 shares of company X stock at $2/share with that $40. Next month the stock prices for company X have gone up and stock now costs $4/share, which means your $40 only buys 10 shares of stock. BUT, those 20 shares that were purchased last month are now worth $80.  Of course, this is a very simplistic example, but should illustrate the point. 

absolutely not! keep putting in. bargain prices now. you'll be in great shape.

Do you have different investment options, so could change to another option. And when you consider the match you're getting, you're still ahead to keep contributing.