You have provided very little information, such as where you live, was this your first tree, the specie, age etc.
The way you have asked the question, my guess is that this was your first bonsai either given to you as a gift, or you purchased it as a treat. Either way you were not in possession of some basic knowledge to care and maintain it.
“I killed my bonsai tree” I hear this a lot and people feel guilty for the tree having died in their care. However, it is not entirely the owners fault. Majority of the the trees such as the one you had are commercially grown and imported. These trees are in already in trouble at the point of sale.
Going back in time to when these trees are harvested from the growing fields in China, they are potted in farm soil and some sand thrown in to aid drainage. The pots tends to be blue in colour and if you look at the bottom of the pot, you will find one or two drainage holes and there is no wire mesh to prevent loss of soil. There are no anchor wires either to secure the tree in the pot. The amount of time these trees spend in these pots after being harvested is upwards of two years and the trees are root bound before they arrive in the West.
The trees are watered, packed in trolleys which are shrink wrapped and then packed into containers. Dry ice is added into the containers and then sealed. It takes up to nine weeks for the containers to get to the west. In the meantime inside the container it is dark and cold and the trees think its winter and go into dormancy.
The containers in the west are unloaded, and distributed to the retailers after cleaning up the pots and adding leaf shine to the tree to make it look attractive.
So the tree you got may still be dormant and also due to being root bound, watering normally would have been difficult, some sellers tell the purchasers to use the dunking method but remain silent on when the trees needs repotting, at best give misleading information on when the next repot is. The reality is that the tree needs repotting as soon as possible using good quality bonsai soil mix and also in a pot which is the right size for the tree.
Different species have different needs to thrive. Some need more water than others and some prefer the soil to virtually dry out before the need for watering. The tree should neither be over watered not underwatered. The soil should not be soaking wet, but moist. In summer months it will need more water and in the winter months less.
Also a lot would depend where the tree is i.e indoors or outdoors. Trees kept indoors need a lot more care and attention than the ones outdoors. Then the tree needs light and sunshine and its location would depend on the specie. Also keep away from heat sources such as radiators.
Age is also relevant as a younger tree has high risk of perishing when neglected than and older tree is sturdy and can survive delays in watering.
Then there is fertilising and that should be done during the growing season. I would suggest a balanced fertiliser 10:10:10 If you cannot get this strength, get the next one up and dilute it down.
I have gained very valuable insight into these bonsai trees as every year there is a constant stream of people I see with their trees in trouble and I have helped save majority of them. I also got to know from the owners where the trees were purchased from and the information they were given at the time of purchase. All this has helped me build a picture of what is happening in this industry. To help people buying these trees, I made three videos and these are in a playlist.
Anyone who is new to bonsai or is given a bonsai tree ought to see these three videos. All you need to know is covered in these videos and the link is :
How to Avoid Common Problems with Bonsai Gift Trees | Bonsai Gift Trees Made Easy Part 1