First, please confirm which model iMac you have. A picture showing the iMac and the condensation will help. As macraider indicated, the last several generations of iMac and the new iMac Pro have the front glass bonded directly to the LCD. If you have an older model iMac with the removable front glass panel (your message is posted in the iMac Pro discussion forum), then it's possible there is moisture trapped between the glass panel and the LCD. I suggest setting the Energy Saver display preferences to never sleep, then run an app that warms the system up and leave it running for several hours to drive out any residual moisture picked up during the three months of storage. One option is to download the free Heaven VR demo from Unigine and let it run. It will exercise the CPU and GPU and warm up the system.
UNIGINE Benchmarks
If the issue returns then I would investigate whether the relative humidity in Malaysia where your iMac or iMac Pro is located exceeds the Apple specifications. For many of the iMac products the following specs apply:
Relative Humidity: 5% to 95% noncondensing
If the moisture in the are is condensing in your iMac then you may need to investigate purchasing a dehumidifier to bring the humidity level down below 95%.
