You should contact the HR department of the company that you applied to and verify that you are still being considered for the position.
There are two possibilities here:
You are still being considered for the position but for some reason your hiring manager is unavailable to contact you. This may be intentional (the hiring manager or someone else is stalling the hiring process for political/technical reasons that have nothing to do with you) or unintentional (the hiring manager or someone else was hit by a bus or is out with the flu).
You are no longer being considered for the position, and they have chosen not to notify you as such. While not notifying finalist interviewees that they are no longer being considered is a rather unsavory practice, it is unfortunately relatively common in the corporate world. The company may provide any number of "reasons" why they might not notify finalist candidates of their consideration status but in the end the message that is relayed is that the company lacks common courtesy.
You should do this as soon as possible. If it turns out that the offer is still on the table, you should notify them that you no longer want to be considered when you accept the other position. It would be rude on your part to let them continue their hiring process believing that you still wanted the job when in reality you have already accepted a position elsewhere.