Can one co-owner lock the other co-owner out?

1) A co-owner may NOT lock the other co-owner (or their guests) out: each co-owner has an equal right to access, use and occupy the home, or have guests in it (like his girlfriend). You can go to court on an "emergent" (think urgent or emergency) basis to get a court order letting you back in. You could do this yourself (pro se) without an attorney if you wanted--the clerk's or customer service office(s) at the court house should be able to give you instructions or forms (they can in the state I practice in: NJ)--but having a lawyer is recommended.
2) You can't go to family court in regards to who keeps the house like you would in a divorce, since you are not married; but you *can* bring a legal action traditionally or historically called an action "for partition" (your state may have a different name for it) in which you seek a court order that the house be sold and the proceeds (after paying cost of the action, costs of sale, and the mortgage, if any) be split or divided among the owners. The court in doing so may take account of who paid how much, so if your ex-boyfriend "paid cash for it," he may the bulk of the money from the sale of the home, to reflect that he paid for it initially. You could explore this option with your attorney.
That you have been living in the home does not give you any particular right to it, since you are not married: your rights are based solely on you being a co-owner, potentially adjusted by the court, as discussed above, for how paid how much.
3) Or alternately: once you go through 1) above and get access, you have leverage over him: if he wants the home for himself and his new girlfriend and wants to keep you out, he'd have to get you to sell or turn over your interest to him. You and he may be able to work out something in which he will buy you out, since the alternative is for him to bring the legal action described in 2), which will take him months and cost him money anyway.