Here are ways to brainstorm some ideas.
1. Find the right time of day. Are you more creative in the morning after a good night sleep or maybe after your workout and got the blood flowing? Maybe it's a certain day of the week like the weekend when you are more relaxed. What ever time of day be aware when you are most creative will help you when you have a creative rut.
2. Take inspiration from imagery. When you look at other ideas can get your creativity juices flowing and create ideas you never thought of. Look at the detail and a various of images. Ask yourself how did they get that effect , what medium did they use, what other medium could be use. Asking your self questions can help look at it in a different way.
3. Keep things judgement free. Make sure when you brainstorm let your mind wander. We tend to judge our own ideas because we do things a certain way, but that can limit you.
4. Switch your position. We can be creatures of habit. So if you usual sit down when you paint try standing up or maybe draw outside on a nice sunny day. When you switch things up it can trigger mental change.
I hope these ideas help you and don't be too hard on yourself, It happens. Just relax have fun, brainstorm out of the box and let your mind take you where it wants. You maybe surprised where it takes you.