Your money is not stuck when you open a money market account and you are able to withdraw your money from a money market account but there are some restrictions on when and how often you can withdraw money from a money market account.
With a money market account you only get six transactions (transfers or withdrawals) per month, or per account cycle of at least 4 weeks.
The limit for withdraws and transactions with a money market account is because of a law which is Regulation D, a federal law that limits transfers and withdrawals from money market accounts.
Also money market accounts are not FDIC insured so you could lose your money in a money market account if the place fails or your investments in a money market account goes sour.
Sometimes a money market account can be a good idea and other times it may not be such a good idea so I would study up and think good before investing your money into a money market account.