In your working life you will often have many moments that make you question your career or the company you are working for. This may be that there have been recent changes that you do not agree with or that you feel have made it harder to do your job or that there have been new management staff bought in whom your simple do not see eye to eye with.
Before rushing in to writing your resignation letter take a deep breath and have a few days to really consider your options. Firstly find out why it is that you are no longer happy, is it something that is likely to change in the foreseeable future? If it is then it may be a case of having to grin and bear it for a while and just see out the bad times. The grass is always greener and often you may find that if you leave suddenly that actually you are no happier somewhere else but make not have all the benefits you had at your previous company.
Often people want to leave their job due to issues with pay and wanting to earn more money. It may be that you have spoken to your employer and they are not willing or able to give you a pay rise and finding a new position elsewhere may be your only option.