While using a mercury thermometer if it breaks and the mercury content in it is accidentally swallowed will it cause poisoning? and will it cause any long term effects or side effects on the body of the person swallowed it?
The mercury found in a thermometer is elemental mercury. It is poorly absorbed from the gastrointestinal tract. However, the greatest problem with it is that, apart from ingestion, there can be another route of exposure without noticing. Elemental mercury easily evaporates on room temperature, and these vapours can be inhaled. In this form Hg is liposoluble and is readily absorbed and passes the blood-brain barrier easily. Since main toxic effects of any form of mercury are neuro-toxic ones, this is a major concern.
There is more information about toxicity of various forms of mercury in this Medscape article.
An EPA Toxicity and Exposure Assessment for Children's Health Report on elemental mercury states that:
Elevated levels of elemental mercury or mercury vapor in indoor air can occur as a result of accidental spills and can be lethal to children. Spills have occurred in home and school environments. Attempted clean-up using a vacuum cleaner disperses elemental mercury into the air, and is likely to increase exposure.
So, yes: exposure to elemental mercury such as the one from a broken thermometer can cause poisoning. It can affect the nervous system, lungs, kidneys... "Recovery is usually without sequela" (without permanent consequences), but there have been cases with fatal outcomes. In pregnant women exposure has been associated with an increased rate of stillbirths.