That alone is already an insanely shady action, because every properly functioning exchange has enough coins in reserve to allow people to withdraw at least relatively smaller amounts without waiting.
If you can't even withdraw Bitcoins valued under $250, then that exchange is doing that intentionally. It could be that they turned insolvent, or they plan an exit scam, where I personally hin at the first mentioned option.
Network congestion is no issue at this point and thus can't blatantly be used as excuse.