I have lost my p45 what shall I do?How do I get my tax code?

It should be written on any old pay slips,,,

Do you have a copy of your old P60s that you woudl have got about every April/May? That will have your tax code on. You can also contact your old employer and ask them for a Statement of Earnings - they can either write this down on paper or print off a report for you, depending on how they work. Or, you can get a P46 from your new employer or tax office and this will act as a temporary p45 and stop you being taxed at emergency tax rate until you get it sorted out.

If you are going to a new job, complete a P46 and you will get your code back or if you have your last payslip take it with you. On it ,it will show your tax code and all the payments made to date.

I have to assume that you are about to start a new job, and therefore require your tax code to ensure that you are not taxed on a week/month 1 or basic rate basis just before Christmas.
Even if you have kept your last payslip, which must contain your tax code, there is no way that HMRC can assume that it is your last pay slip.
Even if you have a letter accompanying your departure from your previous employment, HMRC would argue that you could have entered further employment in the interim (unless your departure was this week/month).
In this case you will need to complete a P46 to state that this employment is "your only or main job". At least this ensures that your gross salary is not taxed entirely at the basic rate (22%), but on a week/month 1 basis which means your are given the personal allownance etc as if your wage slip was the first week/month in the tax year. It will not give you a tax refund that may be applicable if you have not been working for quite a while.
Your previous employer is obliged to send part of the P45 to HMRC within 14 days of your departure. If you contact your previous employer, they may well have a copy. If not, you could ask them for their PAYE reference with HMRC which is structured as follows: xxx/aa/0000xxxx ie 3 numbers, 2 letters, 4 zeros, 4 numbers. If you contact HMRC and state your problem, providing you have your Unique Tax Reference number (10 digits from your self-assessment form), and/or your National Insurance number, they MAY tell you whether your employer has submitted part 3 of the P45. If this is the case, the HMRC will send your new employer your tax code when they receive the P46.
The bad news is that this will probably take some time. The good news is that it cannot take longer than the end end of the tax year.

Your new employer should be able to get this from their local tax office. They will put you on an emergency code for a bit though.

Old payslips do not give your new employer the authority to use the code shown on them. If you do not provide him with a form P45, P46 is the way to go, so that he can use the emergency code. Your old employer is not allowed to issue a duplicate P45 (it's a potential fraud issue), so all you can do is contact the tax office, and ask them to issue a form P6 coding notification showing your code number and previous pay and tax to your new employer.