It is fairly common for exchanges to have (short) pauses in withdrawals. The majority of funds are in a cold storage, and for security reasons it usually takes some time to get the funds out from there "live".

If lot of people are withdrawing at once, it would cause minor inconvenience to any exchange. They have to transfer funds from cold storage, and that takes time. It is almost impossible to determine if an exchange is solvent or not until you know their cold storage addresses.