AirTag location is lost

The battery probably died or there is not another iPhone within bluetooth range for the AirTag location to be updated. If someone finds the AirTag and removes the battery, there also will not be a way for it to be found. Those scenarios and more are listed here:

Use Find My to locate your lost Apple device or AirTag - Apple Support

muslim41 wrote:

My new AirTag is showing distance 152km last location has been showing in the same place for 7 days.what to do now

An AirTag has no method by which to detect its own location (it doesn't have GPS), or directly report/transmit its movements.

An AirTag is effectively little more than a "beacon" that broadcasts its unique identity using Bluetooth, this beacon signal being detected by other Apple devices such as an iPhone.

When an iPhone with a data connection in near proximity detects an AirTag (which will typically be within 10m/30'), the iPhone will anonymously and securely report the detection of the AirTag's unique ID to Apple's back-end servers, along with the location of the iPhone. As the AirTag will be within a short distance of the detecting device (for which the location is known), the location of the AirTag can be inferred with high confidence.

If your AirTag's reported location has not moved, this would suggest that:

a) the AirTag has been detected by other devices in close proximity, but the AirTag has not been moved from its last reported location - or

b) no devices capable of detecting the AirTag have since been within 30m/30' of the AirTag to detect and report the location of the AirTag - or

c) the AirTag's battery has failed or has been removed.

Use AirTag and Find My to keep track of your personal items - Apple Support

What is attached to that AirTag? Why are you asking what to do? We’re not mind readers here. Sounds like you need to contact the police. If something was stolen then do not attempt to recover it yourself. You could be in serious danger if you confront a thief on your own.