This is a question that cannot be answered without knowing a considerable amount more detail. Your rights depend on the terms of your employment contract, the employer’s motivations in your termination, and other factors. Normally, any severance package should properly include an amount of money in a lump-sum to put you in the same position you would have been if you had been given reasonable working notice, rather than fired without notice. This depends of course of the terms of your specific agreement, and any implied terms which are not superceded by a written contract.
The amount of the severance package should compensate you for all losses, which would include salary, amounts paid to replace benefits that were cancelled, often, an amount to compensate you for any bonus you would have earned during the notice period, an amount to compensate you for the loss of enhanced pension benefits from not being able to continue to work, losses from failure to have additional stock options vest, and a longer period to exercise options, and other similar losses. In addition, your severance package should include amounts due to you for your service up to the date of termination, including bonus for prior periods, commissions earned for sales made while working, an amount to compensate for vacation not taken, and the like.
The best advice is that you should call us in order to be sure you fully understand your rights before accepting a severance package.