The crown may have fell off due to the use of chewing gum or due to sticky foods.Your tooth did hold the crown for 2 years and just didn't come off early within a month or so suggests of two things, either the preparation of tooth does not conform to retentive and resistance forms of tooth preparation or the tooth structure is insufficient to hold a crown.Now that your case has been complicated i suggest you refer to a specialist. In Case you have sufficient crown height and the tooth preparation is inadequate, few changes in the preparation by maintaining parallel walls and addition of a groove will hold the crown for long.In the situation if you don't have adequate crown height the tooth crown height can be increased by crown lengthening procedures or by placement of post and core,I would personally not suggest extraction of tooth if there is no periapical pathology or if RCT achieved a good apical seal.
Next Steps
Refer to a prosthodontist.
Health Tips
Avoid sticky foods and chewing gum after placement of crown.The crowns can withstand good compressive forces but have poor tensile strength.