Assuming there's still some trade going on, with Bitcoin being traded at much lower levels due to little demand: The hashrate would be much lower, but Bitcoin would continue running as is, meaning it would still be usable -- assuming you find someone to exchange Bitcoin for fiat, goods or services with. The good news is, transactions would be incredibly cheap as the blocks would be near empty and what little sat/byte you'd pay in transaction fees would be worth near zero in fiat terms. The bad news is, once block reward dries up and close to no transactions take place, mining would indeed likely simply cease to exist beyond some hobbyists, if that.

If there were literally no trade going on, as in zero liquidity and no one being willing to accept Bitcoin as a means of payment -- then no, Bitcoin would not be usable anymore and the theoretical price would be frozen at whatever valuation the last known trade happened. The market would have no way to react to it, as by definition, no market would exist.