It depends when you are 50 for a start - if it is after April 2010 then the minimum retirement age is moving to 55 - so that is the first point of call.
Second - you cannot sell your pension - this is not an option on a pension.
Your options are - leave the money where it is, and let is grow over the next few years, or you could transfer it to another provider if the fund has not been growing well since you last paid into it. If you have a company pension scheme, this may take the transfer and this may be adventageous as it may purchase so extra benefits.
If you are eligible to take the money, the most you can take out of a Personal Pension is 25% of the fund as tax free case - so if there is £50,000 in the fund you could have £12,500 as Tax Free Cash and use the rest to purchase an annuity, which would give you an income.
This is not the only solution but without knowing lots more details it is hard to say.
I would approach an IFA as your first point of call and take this from a recommendation as this is always best!
Hope this helps!