There are some things that you need to check first:
1. What temperature is there in the room that you are fermenting in? Some yests does not like it below 13 and some dont like it above 17-18. Usually you can get it to start by adjusting this.
2. How much air is there in the fermenting container? If you have a lot of air it can take quite a bit of time before the preassure builds up, it can give you an off flavour with too much air so be mindfull of this.
3. How much yest did you add? If there was not much yest added it will take quite a while before they have multiplied and started to convert sugar into alcohol. And also, as usual when fermenting, it can give a bit of and off flavour. Having a starter can be helpfull to ensure enough yest cells are added. If you have more you can try to add it after a day or two.
4. Sugar content very high/low. In my experience, there are some yests that dont like when there is a very high sugar level. They simply dont start to ferment (or will give a rather bad taste). If you want to make a really strong beer or mead, check which yest that likes a high sugar level and can survive with a lot of alcohol.
If you have checked these things and it still doesnt start to ferment you might have to add some nourishment to the beer to help the yest. There might also have been a contamination, meaning that your equipment wasnt clean enough and other bacterias have gotten into your brew. If that has happened you can just as well pour it out, it is ruined.