If you are continuously facing this problem then, it may be due to the ill functioning of your contacts app. To resolve the problem download a new app from playstore, which can store all your contacts.
Sometimes it may happen that your settings are not up to the mark, in that case try the below provided method.
1. Open the "Contacts" app
2. Press on the menu in the top-right corner (three vertical dots)
"Contacts to display"
3. Select "All contacts" if it isn't already
If "All contacts" hasn't been selected, then this was probably the solution to your problem. However, if all contacts were already displayed, lets try something else.
Try to backup your contacts so the next time if you loose your contacts. You can restore them from there.