If I understand your question right, when you were hired, you were awarded options to buy a certain number of shares of stock at a certain price. Those options vested over time. So, for example, perhaps you were awarded options to buy 48,000 shares of stock, vesting monthly over 4 years. So, after 1 month, you could buy 1,000 shares, after 2 months, 2,000 shares, etc.... (Or, alternatively, after 1 month, you could buy 48,000 shares, but the company could repurchase 47,000 of them at the same price you bought them for.)
What happens depends on the terms of your option. So, you have to look at your option agreement and the terms of the option plan.
There are two likely scenarios:
(1) Upon the acquisition of your company, the vesting on your unvested shares is accelerated, so you can exercise the full amount immediately prior to the acquisition. If that happens, then you get whatever a shareholder would get -- cash, the acquireror's stock or a mixture of both.
(2) Your options are exchanged for options to buy shares of the acquiring company.
Now, if the acquiring company is buying your startup for cash, chances are they will just want to buy you out -- there's often a provision in the plan to allow them to do that.
