Everyone knows "the spaghetti print", and every time it happens it's equally disappointing. When a print breaks loose from the buildplate while printing, it can have catastrophic consequences for the print. When the printer has many hours to spit plastic into thin air, it usually turns into a large pile of plastic that can do little more than end up in the trash. To avoid this, you can make check up on the following points before you start printing.
Check build plate heat
Most 3D printers come with a heated build plate these days. This feature is important for prints to stick to the plate, as most surfaces that printers come with are designed to grip the print when it is hot, and release when it is cold. Make sure your slicer sets the temperature in the start GCODE of your print, and that your printer warms up the plate before the print starts.
If the build plate is being heated, try increasing the temperature by a few degrees.
Level your bed
The most important layer in any print is the very first. If the distance between the nozzle and the plate is not spot on, the plastic may have difficulty sticking to the plate and will have a high probability of popping off the plate along the way. The optimal first layer is made by adjusting the height so that each line is squished out with the tip of the nozzle, so that there is no gaps between the lines. This provides the largest surface area between the part and the buildplate.
Clean the build plate
All dust and particles that could be trapped between the plastic that the printer places on the plate, and the plate itself will contribute to a more loose model grip. If the printer has been idle for a week or longer, it may be a good idea to wipe the plate with a damp cloth.
It's also very normal that the plate over time will need to be wiped off with rubbing alcohol, which can dissolve fats. When you touch the plate with your fingers, you leave natural oils that your skin makes. The plastic has a very hard time sticking to these oils, and therefore it's important to avoid touching your build plate unnecessarily - otherwise the alcohol must come out more often.
Ensure the model lies flat
Some models you download from the web may not be completely finished or otherwise not quite 3D printer friendly. It may happen that the model to be printed doesn't lie completely flat on the plate in the slicer. When the slicer tries to slice your model, it doesn't know that it is not meant to lie like that and it will just slice it as is. If possible, rotate the model so that it lies flat on the plate.
The smart rotate feature in the SimplyPrint slicer ensures this should never be a problem. Alternatively, in the SimplyPrint slicer, you can hold down CTRL and click on the side of the model that should face down towards the plate.