It is normal for human beings to occasionally doubt things they cannot see directly.
There are many reasons for this.
* It could be a phase or part of your own personal religious/spiritual growth, as part of expanding your concept of the nature of God.
* The materialism and busyness of the modern lifestyle can drag people down to focus on worldly issues only
* Sometimes, things we do (such as certain sins) or certain things such as overeating, overly watching television, or anything else that makes us overly like a vegetable can also interfere with our "spiritual hardware" that makes us aware of what is beyond this.
It could be all of this or none of it.
Anyway, since your concern is specifically over whether God exists, you could:
* Spend time reading books and watching videos discussing the existence of God. There are a lot of valuable materials both by Muslims and also people of other faiths.
* Spend time reading the Qur'anic discussions about God - keeping in mind that many of them were aimed at people who didn't believe in God
* Read hadith about the nature and existence of God (you could probably find some on this website)
* Prayer - one of the most personal ways that people come to appreciate the existence of God is through answered prayers. I came across someone once who empirically tested prayer and made lists of prayers that he did and when they were answered, and he concluded after a year or so that prayer was effective. Although God is above being tested, it doesn't hurt to try!
* Prayer for guidance - If you ask God to prove to you that He exists, your prayer will likely be answered.
* Spending time in nature, if it is possible, is also a good opportunity for contemplating on spiritual matters. After all, one of the main arguments for the existence of God put forward in the Qur'an as well as by philosophers is the order and beauty of the natural world. Sadly, many of us are separated from it today due to living in cities and other things and we do not always appreciate what is out there.
I am sure you will find your way - take this as an opportunity to explore a very important question of theology (the existence of God) in-depth and become an expert on it, rather than just taking it for granted!